I have a developer workstation that has registered DDR2 ECC DIMMs already installed. I'm planning on getting some more RAM. Can I add memory modules that aren't ECC and/or registered? I mean, does the machine work with both ECC and non-ECC modules installed? I guess registered ECC memory is at least more expensive?
ECC and non-ECC can be mixed on the same motherboard if the motherboard does not require ECC. (Funny, I just tried it this morning.) Your BIOS also has to be able to force the ECC to operate in non-ECC.
Do you know if the motherboard requires ECC? What's the model?
I had this problem with an HP server in the past. You can make it work, but it is a hit or miss thing. It is very dependant on the memory modules being mixed. Some work together, some do not. I mixed and matched a few different combinations. Some did work, others did not. In the end I decided to cut my losses and just move forward because the larger modules I had did not play well with the system. In my case I bought non ECC memory to replace the small sticks of ECC the system came with. They gave me a whole 256 MB of ram with that server so keeping the ECC was hardly worth doing. The new memory worked fine and the only thing that it did was complain that non HP memory was being used.
My advice is just buy a uniform memory module type once you find one that works. Ditch the ECC and just replace all of it. You could have anomalies due to the variances between the sticks and their performance capabilities.
I have heard it will work if the motherboard is ok with it, but it is not advised or recommended by anyone. Using mixed RAM also defeats the purpose of using ECC Ram in the first place though.